1. Field of the Invention
This invention relates to drilling machines and more particularly to machines utilized to drill printed circuit (PC) boards or the like. Specifically, this invention relates to the detection of broken bits in such drilling machines.
Printed circuit boards generally require holes to be drilled in them with high accuracy with regard to size and spacing. The drilling is usually done in a gang fashion on a machine having a plurality (typically three, four or eight) of individual drill spindles. The number of PC boards drilled at one time is equal to the number of spindles in the machine, i.e., only one hole at a time is drilled in an individual PC board. Since the machines operate at speeds of up to 400 cycles per minute, the drill or PC board must be repositioned extremely quickly between drilling cycles. The positioning of the drill spindles with respect to the PC boards is generally computer controlled.
Typical hole sizes which are drilled by the PC board drilling machines range from 0.008 inches to 0.25 inches. The combination of high drilling rate and small drill bit size occasionally results in a drill bit being broken. Because of the fact that several spindles are usually operating simultaneously and because of the high operating speeds employed, it is very difficult for an operator to observe when a drill bit is broken. Although breakage is relatively rare (on the order of one in every 20,000 holes which are drilled), it can be very costly because the entire PC board must be scrapped. This is especially so in the case of highly critical, extremely expensive PC boards such as are used in certain computers. Therefore, it is very desirable to have some method of accurately determining when a drill bit has been broken so that it can immediately be replaced.
2. Description of the Prior Art
Most drilling machines presently in use include a pressure foot which is movably connected to each drill spindle and presses down against the PC board to clamp the board in a fixed position for each drilling operation. The pressure foot includes a central opening through which the spindle chuck and drill bit extends. Although the pressure foot aids in obtaining accurate location of the holes, its presence makes it more difficult for the machine operator to observe the drill bits. In order to provide broken bit detection in spite of the masking presence of the pressure foot, attempts have been made to utilize light beam detection systems. In such systems, the drill bit lies in the path of the light beam which is directed towards a light sensor. Normally, a drill bit breaks the path of the beam and prevents the sensor from being activated. However, if the drill bit is broken, the beam will not be blocked and the sensor will be activated, thus indicating the absence of a complete bit. Such light beam systems have not been successful because they are quickly worn out by being hit by flying abrasive particles created during the drilling operation. The abrasiveness of these particles is such that the light emitter and sensor elements had to be replaced almost weekly.
In view of the foregoing, it is the primary object of the present invention to provide a broken bit detector for PC board drilling machines which is both accurate and rugged.